None.
Generally, tire uniformity machines are used to test the characteristics of tires after production. This testing includes measurement of the force and dimensional characteristics of the tire at varying loads. To accomplish load testing, the tire is brought into the tire uniformity machine, mounted on a chuck, and rotated by a drive assembly coupled to the chuck spindle while contacting a load wheel.
In a typical tire uniformity machine the load wheel is rotatably supported on a carriage that may be brought into and out of contact with the tire. In one known tire uniformity machine, the carriage has a pair of spindles rotatably supporting the load wheel with load cells corresponding to each spindle placed in sensing relation thereto. The load cells have an external rim that is attached to the carriage and a sensing portion located within the rim that is attached to the spindle. A cover is typically attached to the sensing portion and used to protect the sensitive internal components of the load cell. Forces transmitted through the spindle cause the sensing portion to emit an electrical signal, via a pickup on the load cell, to a controller or display unit. From the reading generated by the load cells and data provided by other sensors, the controller determines the tire""s characteristics.
Due to the extremely sensitive nature of the load cells, when setting up the tire uniformity machine, it is important to properly calibrate these load cells with each other to obtain accurate readings. In some uniformity machines only a single load cell is readily accessible to the operator. Thus, to calibrate the load cells, the inaccessible load cell is attached to the carriage as previously described, and the accessible load cell is moved relative to the fixed inaccessible load cell to obtain calibration. To calibrate load cells, tie accessible load cell is moved such that it may be attached to the spindle. Once this load cell is properly located, the axis between the accessible and inaccessible load cells must be aligned.
Typically, alignment of the axis is not performed. Often the alignment is assumed from the orientation of the load cells. Without proper alignment of the axis, the load cells produce inaccurate readings. For example, in measuring forces at the spindle, the forces generated may be supplemented or reduced by the inaccurate readings of the load cells. While this altering of the measured force may be on the order of a tenth or single pound, the required high degree of accuracy of these machines may result in significant error.
In light of the foregoing, it is an object of the present invention to provide a method and apparatus for calibrating load cells in a tire uniformity machine.
In view of this object, the present invention, therefore, provides a calibration fixture for calibrating load cells in a tire uniformity machine, the tire uniformity machine having a carriage rotatably supporting a load wheel on a spindle, a first load cell and second load cell in sensing relation to the load wheel and attached to the carriage, at least one of the load cells having a support portion and a sensing portion rotatable within the support portion, the calibration fixture including a fixture flag releasably attached to the sensing portion of the load cell; and an adjustment assembly supported by the tire uniformity machine operable with the fixture flag to cause movement of the sensing portion relative to the support portion.
The present invention further provides a method of calibrating a load cell used to measure forces on a load wheel in a tire uniformity machine for testing a tire having a carriage rotatably supporting the load cell on a spindle supported on the carriage, wherein the load cell has a support portion surrounding a sensing portion of the load cell and a cover located within the rim and covering the sensing portion, the cover being fastened to the sensing portion by cover fasteners such that the sensing portion may be rotated by rotation of the cover, the load cell being secured to the carriage by cell fasteners located at the support portion and a center fastener passing through the cover, the method including providing a calibration fixture having a fixture flag and an adjustment assembly operable with the fixture flag to rotate the sensing portion of the load cell; aligning the load cell with the load wheel and fastening the load cell to the carriage; fastening the fixture flag to the cover; zeroing the adjustment assembly; performing a conicity test on the tire noting the conicity reading of the tire uniformity machine; and when the conicity reading is not equal to zero, loosening the cell fasteners, actuating the adjustment assembly to zero the conicity reading and subsequently tightening the cell fasteners.
Therefore, production of a load cell conicity calibration method and apparatus of the type above-described becomes the principal object of this invention with other objects, thereof becoming more apparent upon a reading of the following brief specification considered and interpreted in view of the accompanying drawings.